Can't use Kingston RAM

Back in Nov '05 I purchased a stick of 1GB Kingston Value RAM and was able to use it for about a month or so before I started noticing my system was constantly rebooting. I did a mem test on each piece of RAM and found out it was defective. Returned it to Kingston for RMA and just recieved a brand-new replacement, installed it and wasn't able to boot up to Windows...although it was recognized memory in the bios. I'll admit I did not perform a mem test on the replacement piece, but I just need some advise as to where to begin troubleshooting this mess... each boot-up with the replacement RAM is worse than the one before. I'm suspicious that this has something to do with my hodge podge mixture of various RAM brands and sizes... Please HELP!!!